The present invention relates generally to a phase-shifting transformer system, and more particularly to an improved faster phase-shifting transformer system. The present invention also relates to a method of retrofitting a preexisting phase-shifting transformer system for faster response than in earlier systems, and a method of applying a desired phase shift to the voltage on a polyphase transmission line.
Phase-shifting transformers have been used since the early days of three-phase alternating current (AC) power networks. The basic phase-shifting transformer configurations were established more than sixty years ago, with the succeeding generations of design refining the major transformer components. Phase-shifting transformers are often used to track very slow system changes, on the order of minutes to hours, corresponding to utility load variations. For example, these earlier systems may cycle over a range of 20.degree. in 24 hours, such as by starting at a 20.degree. phase shift at midnight, increasing to 40.degree. during the day's peak load, and returning to 20.degree. again at midnight. These earlier systems were incapable of being controlled for faster operation, and thus, were incapable of forming a part of a stability-enhancing scheme for a power network. Two of the earlier configurations for coupling a phase-shifting transformer with a power network will be described hereafter with reference FIGS. 8A-10 of the drawings.
In general, a phase shift is obtained by extracting a line-ground voltage from one phase of a transmission line and injecting a portion of the extracted voltage in series with another phase of the line. Typically, this is accomplished using two transformers, with one in shunt (referred to as the "regulating" transformer), and one in series (termed the "series" transformer) with the transmission line. This may be accomplished in several ways, two of which are shown in the power circuit single phase diagrams of FIGS. 8A and 9A. FIG. 8A illustrates the basic elements of a one-sided phase-shifting transformer system, while FIG. 9A illustrates the basic elements of a symmetrical system.
Appropriate wye-wye and wye-delta connections of the regulating and series transformers are used to supply the injected series voltage V.sub.S in quadrature with the line-ground voltage. This is often accomplished using a rotated delta connection of the series transformer excited winding, where, for example, phase A of the excited winding is connected with phase C of the regulating winding. This quadrature relationship is shown in the phasor diagrams of FIGS. 8B and 9B which correspond to the circuits 8A and 9A, respectively. In FIG. 8B, the injected voltage phasor V.sub.S is perpendicular to, that is in quadrature with, the incoming voltage phasor V.sub.1. Similarly, in FIG. 9B, the horizontal voltage phasor V.sub.S is in quadrature with the vertical line-ground regulating voltage phasor V.sub.R, which is extracted from the center tap of the series transformer series winding. The phase shift angle in both FIGS. 8B and 9B is indicated as .phi..sub.PS. FIGS. 8C and 9C are single line diagrams for positive sequence modelling of the circuits of FIGS. 8A and 9A, respectively.
The effective reactance of the phase-shifting transformer system varies with the tap setting of the load tap changer (LTC). In the extreme case of zero phase shift, the leakage impedance of the series winding will remain in the transmission path, whereas at the full rated phase shift, the effective impedance will be increased by the impedance of the regulating transformer.
FIG. 10 illustrates one phase of a typical switching network used in the FIG. 8A one-sided and FIG. 9A symmetrical arrangements, with the regulating winding of the regulating transformer shown schematically for convenience as being a part of the switching network. The phase shift is controlled by adjusting a load-tap changing device or load-tap changer (LTC) and by toggling a reversing switch. One end of the regulating winding is connected to the excited winding of the series transformer, as shown in FIGS. 8A and 9A. The wiper on the LTC can traverse the full length of the regulating winding to provide a variable series voltage, with the voltage steps determined by taps on the regulating winding. In this manner, the LTC is used to change the voltage applied to the series transformer excited winding by coupling selected different taps of the regulating winding to a neutral ground potential, i.e. the neutral tie of the three phase regulating winding wye connection.
By toggling or throwing the reversing switch of FIG. 10, phase shifts of the opposite polarity are obtained. However, such toggling action does not simply reverse the sign of the phase shift. Rather, the change in the phase shift angle .phi..sub.PS is equal to the rated phase shift of the phase-shifting transformer set. For example, if the transformer set is rated for a 45.degree. maximum phase shift and the LTC is adjusted for a 15.degree. advance, then throwing the reversing switch will change the phase shift to a 30.degree. retard angle, rather than a 15.degree. retard.
However, one serious drawback of the FIG. 10 switching network, and thus of the arrangements of FIGS. 8A and 9A is the relatively slow action for which both the LTC and reversing switch are usually designed. Additionally, the LTC must change taps under full load, and thus, disadvantageously requires frequent maintenance.
In general, switching of the phase shifter tap positions by moving the LTC wiper across the regulating winding affects both the series flow of current in the transmission line and the shunt voltage at the bus. To bypass the current flow through the series winding, either the regulating or series winding is shorted, which appears as a short circuit on the bus. The short circuit current is limited through the impedance of the regulating transformer, with this impedance selected by the particular tap position of the LTC. On the other hand, if either the regulating or series winding is opened for some reason, this action appears as an open circuit to the transmission line.
Due to these effects on the transmission line, the phase-shifting transformer system switching control must be rather sophisticated to prevent undesirable disturbances on the power system. Existing technology for the LTC function includes vacuum interrupters to break any arc formed when the sliding contacts (wiper) move across adjacent tap positions. This operation of the LTC forms a part of a complex series of internal switching operations.
Relatively newer LTCs have been equipped with thyristor valves to provide arcless operation, which would also beneficially reduce maintenance as a result of less damage being incurred through switch arcing. In these earlier LTC designs, the thyristor valves do not carry current normally, but only when a tap change is taking place. This aspect of the operation has one of the most desirable attributes of a controllable device on a power grid, in that the losses are low during normal operation.
Another type of phase-shifting transformer system has been proposed which has no series transformer, but rather only a specially designed regulating transformer coupled with a bank of thyristor switches. The primary windings of the regulating transformer are coupled with the transmission line, and the secondary windings are coupled with the bank of thyristor switches. The secondary windings are proportioned in ternary progression, that is, they are wound as three separate individual coils, with the respective turns ratios of the first, second and third coils being one, three and nine, respectively. By connecting the three secondary coils in their various combinations, the turns ratio with respect to the primary may be varied from a positive 13 to a negative 13. For example, by coupling a negative polarity of the first coil (turns ratio of one, thus, "-1") in series with a positive polarity of the second coil (turns ratio of three, thus, "+3"), and omitting the third coil, the effective turns ratio of the secondary windings becomes +2. However, since this system has no series transformer, the bank of thyristor switches in series with the transmission line. Thus, this thyristor bank continually imposes a load on the transmission system. Furthermore, the thyristor bank must be designed to handle the full load current of the line, as well as having the same BIL (basic insulation level) rating as the line, both of which are costly disadvantages, in terms of both initial manufacturing costs and operating costs. Additionally, such a one-sided arrangement is inherently more costly than a symmetrical arrangement for large phase shifts. Moreover, this approach requires a totally new transformer design having three secondary windings, rather than a conventional single winding with plural taps. The optimization of such a new transformer design is often a costly and time-consuming process. This system is also an all-or-nothing approach, having no flexibility in terms of selecting various modulation and/or thyristor control schemes to tailor the degree of available control as desired for different applications.
Thus, a need exists for an improved and faster phase-shifting transformer system and a method of retrofitting a preexisting phase-shifting transformer system for use in industrial and electric utility applications, which is directed toward overcoming, and not susceptible to, the above limitations and disadvantages.